[rollei_list] Re: Contrast and Resolution
- From: "Neil Gould" <neil@xxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Fri, 9 Nov 2007 09:54:12 -0600
Hi all,
Wow... thanks to all for your contributions on this topic! I've printed
this digest out, and will consume it in depth. Perhaps then what has
already been written will complete my enlightenment about these factors,
but knowing how quickly these threads dissolve, I would like some
clarification based on my immediate reaction to some of your responses.
> Date: Thu, 8 Nov 2007 11:31:44 -0300 (ART)
> From: Carlos Manuel Freaza <cmfreaza@xxxxxxxxxxxx>
>
> The lens resolving power is the ability of the lens to
> determine fine detail, for a traditional photographic
> image, the image resolving power is a combo between
> the lens and film resolving power.
> The lens contrast has to do with tonal values, the
> lens contrast is the ability of a lens to distinguish
> differences for very similar or almost identical tonal
> values in the image.
> The point in the Zeiss paper published in the Zeiss
> info No 51 is that the human eye distinguishes as the
> best image the image with the best contrast and not
> the image with the best resolving power, they show
> very clear examples about it but the images are not
> on-line.
>
[...]
> The lens resolving power is significant for the image
> quality, but more important and more significant is
> the lens contrast, this is the Zeiss point for this
> article.-
>
Thanks for the explanation, Carlos. So far, this has been my
understanding, particularly with the introduction of "fine details" into
the discussion. However, my puzzlement has more to do with the ability to
resolve such details without contrasting surrounding tonal values than the
perceived "best" quality of an image.
> From: "Eric Goldstein" <egoldste@xxxxxxxxx>
>
> Neil -
>
> This is the flaw in your thinking... what leads you to believe that a
> lens with lower contrast will necessarily have lower resolution?
>
I'm trying to understand how one can resolve details without the
surrounding contrast.
> Also, have you ever photographed a lens test chart and examined the
> resulting negative under a microscope? In the times Ed was discussing,
> this was one of the common procedures for testing resolution sans
> contrast.
>
I understand this procedure, but it doesn't clarify what is meant by
resolution sans contrast.
> From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
>
> The most common measurement for resolution is a
> modulation transfer graph arranged to show resolution vs:
> contrast. The contrast being the that between the dark and
> light lines. The curve will show the effect being discussed.
> It is a misunderstanding to think the "contrast" being
> discussed here is overall image contrast, rather it is the
> contrast between the light and dark lines. This is
> essentially the same as the MTF curves shown for film.
>
> For a "high contrast" lens the MTF chart will show the
> contrast staying at a high level into the mid range of lines
> per unit measure and then falling off fairly rapidly. For a
> high resolution lens the contrast at mid values will usually
> be lower but the curve will fall off slowly and extend
> further than the high contrast condition. At very high
> values of resolution the high resolution lens will have
> _higher_ contrast than the "high contrast" lens.
>
This has been my understanding of the methodology and related issues, and
the main reason that I'm unclear about the notion of resolution sans
contrast. In the above, they seem to be directly rather than inversely
related.
> The difference in the two conditions depends on how the
> designer has balanced the higher order aberrations and on
> what I will call the illumination of the entrance pupil.
> If one charts the energy distribution of a beam of
> light focused by the lens one finds that the high contrast
> lens concentrates the light in a small beam or spot but has
> many smaller peaks or beams surrounding the main beam. These
> cause a sort of flare around the main beam lowering the
> contrast. By suitable adjustment these secondary beams can
> be much reduced at the cost of making the main beam broader.
>
Out of curiosity, does the broadening of the beam obliterate some near-by
details? If so, is that a reason for suggesting an inverse relationship
between contrast and resolution?
I appreciate your clarification of the trade-off between high resolution
and high contrast designs. There is much food for thought! Thanks again,
for such detailed responses!
Regards,
Neil
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